(1) Field of the Invention
This invention relates to plasma arc welding (PAW), and more particularly to plasma arc welding apparatus having optical enhancement and monitoring of the location where the metal is being fused so that accurate, real time information is available for controlling the weld.
(2) Description of Related Art
Plasma arc welding is a process wherein coalescence of metals is produced by heating them with a constricted arc between a nonconsumable refractory electrode and the work or between the electrode and a constricting nozzle having an orifice. A filler metal may or may not be added during the process. Thus, plasma arc welding is a method of electrical arc welding in a protective gas wherein the arc is maintained between the workpiece and a nonconsumable electrode of a high melting point metal such as tungsten, the plasma of the arc being passed through a nozzle. The electrode is disposed within and insulated from a welding torch body and a front end having the constricting nozzle. An inert gas is supplied to the torch body and is channeled about the electrode exiting through the orifice. This gas acts upon the electric arc to constrict its shape to that of a narrow column and becomes ionized in the arc to form the plasma. In addition the orifice gas provides some shielding effect to the molten weld zone and the electrode. In most, if not substantially all, cases an auxiliary shielding gas is provided by channeling a second inert gas stream to blanket the area of arc plasma impingement on the workpiece to avoid contamination of the weld pool.
In plasma arc welding of certain ranges of metal thicknesses, certain combinations of plasma gas flow, arc current and weld travel speed will produce a relatively small weld pool with a hole, known as a keyhole, penetrating completely through the base metal. In a stable keyhole operation, molten metal is displaced to the top bead surface by the plasma stream to form the characteristic keyhole. As the plasma arc torch is moved along the weld joint, metal melted by the arc is forced to flow around the plasma stream and keyhole to a progressively lower temperature area at the rear where the molten metal is solidified. The motion of the molten metal and the complete penetration of the metal allows the impurities to flow to the surface and the gases to be expelled more readily before solidification. Orifice gas flow rates for PAW welding are critical and must be closely controlled in the keyhole mode.
The high density arc column in PAW in the keyhole and non-keyhole modes require accurate alignment between the weld torch and the joint seam to assure weld joint integrity. Structures and vessels fabricated with this process require high tolerance in joint fit-up and rigid tools for maintaining critical alignment of the parts during the weld process. Monitoring the process is a full time task by either a welder at the torch or at a remote location using a video monitor. Current systems in use and in the development stages utilize various methods to locate the weld joint seam. These methods can be inaccurate and easily rendered unreliable resulting in costly errors.